Splitting a string in C++ (using strtok?) separated by a comma deliminator and using no external libraries?

Question

I'm really new to coding and I'm having some trouble with trying to split strings in C++. I'd like to know how to split a string, which is input as a const char names[] (i.e. "Mary, Jan, Jane") in C++ without using any external libraries (i.e. I don't want to have to use #include <string> etc. - although I can use #include <cstring>).

I've tried using:

const char names[] = "Mary, Jan, Jane";

char *token = strtok(names, ",");
while (token != NULL) {
    token = strtok(NULL, " ");
}

But I can't seem to pass in a const array of chars, and I'd also like to know then how you would access all the individual "tokens"?

ALSO I've tried changing the input to just char names[] (but I do need the input to be const), and I get a segmentation error and I don't understand why.

`strtok()` doesn't take a constant for an argument. It actually modifies the string passed in. — Andrew Henle, Oct 26 '17 at 20:08
*without using any external libraries* -- So what do you consider `strtok()`? — PaulMcKenzie, Oct 26 '17 at 20:12
Why can't you use #include ? Is this some kind of homework assignment with that requirement? If so, I'd question that quality of my professor... — Christopher Pisz, Oct 26 '17 at 20:17
When you talk away the `const`, did you also take away the `[]`? — David Schwartz, Oct 26 '17 at 20:17
Can you explain more clearly what you actually want to do? Do you want it to copy the tokens out of the string? Do you want it to give you the start and stop positions of the tokens in the string? Or what? — David Schwartz, Oct 26 '17 at 20:18
*I'm having some trouble with trying to split strings in C++* -- Maybe it would have been better to change the code to `C` and ask in the `C` group on StackOverflow, since you want to throw away C++. — PaulMcKenzie, Oct 26 '17 at 20:18

Ron · Answer 1 · 2017-10-26T20:35:14.133

5

Use std::string instead of char arrays and utilize the std::stringstream class. Pass the , delimiter to a std::getline function:

#include <iostream>
#include <string>
#include <sstream>

int main() {
    std::string names = "Mary, Jan, Jane";
    std::string temp;
    std::istringstream ss(names);
    while (std::getline(ss, temp, ',')) {
        std::cout << temp << '\n';
    }
}

The only thing left is to handle the leading space character in each of the strings:

if (temp.front() == ' ') {
    temp.erase(0, 1);
}

edited Oct 26 '17 at 20:35

answered Oct 26 '17 at 20:12

Ron

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Is there an alternative? I can't use the library – J Dope Oct 26 '17 at 20:13
1

Have you run this through a performance test compared to strtok or boost::algorithm::split? It might be worth noting, if the bit of code in question is to be performant. – Christopher Pisz Oct 26 '17 at 22:04

alain · Answer 2 · 2017-10-26T20:18:49.207

1

strtok modifies the input string. The string is passed as a char*, not a const char*. It replaces the delimiters by '\0'. This makes it incompatible with a string literal, which is a const char*.

I would never use this function, because it also maintains an internal state, which makes it unuseable with multiple threads or even calling other functions that also use strtok while you are in a function that uses strtok.

If you want to see some (many, actually) ways to split a string in C++, take a look at this Q/A:
The most elegant way to iterate the words of a string

edited Oct 26 '17 at 20:18

answered Oct 26 '17 at 20:12

alain

11,939
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what would you use instead though? – J Dope Oct 26 '17 at 20:17
I added a link. – alain Oct 26 '17 at 20:19

score 1 · Answer 3 · answered Oct 26 '17 at 20:16

The correct usage of strtok() is as follows:

int main ()
    {
      char str[] ="Mary, Jan, Jane";
      char * pch;
      pch = strtok (str," ,");
      while (pch != NULL)
      {
        printf ("%s\n",pch);
        pch = strtok (NULL, " , ");
      }
      return 0;
    }

score 0 · Answer 4 · answered Oct 26 '17 at 20:14

My preferred method is to use boost::algorithm::split

http://www.boost.org/doc/libs/1_63_0/doc/html/string_algo/usage.html#idp11326066

It is safer then strtok, which is a C function expecting c-strings.

It will require you use std::string and a container such as std::vector.

You mention you don't want to use

#include <string>

I'd question: why not? If you are going to be programming in C++, being familiar with std::string and the STL is going to be essential.

If you are new to coding, you might not be familiar with boost yet, but it is one of the most commonly used libraries in C++ there is. In fact, alot of what goes into the standard comes from boost.

Splitting a string in C++ (using strtok?) separated by a comma deliminator and using no external libraries?

4 Answers4